This invention relates to facsimile systems comprising a transmitter, a receiver and a communications network therebetween. More particularly, this invention relates to a system where a document is scanned in a facsimile transmitter to generate electrical information-bearing signals representing dark-light variations in the document being scanned. These information-bearing signals are transmitted over the communications network to a facsimile receiver where the information-bearing signals are converted to marks or images on the copy medium so as to form a copy which is a facsimile of the original document.
Typically, the document at the transmitter circumscribes a rotatably mounted drum and a copy medium at the receiver similarly circumscribes a rotatably mounted receiver drum. The transmitter drum and the receiver drum may then be driven at substantially the same angular frequency, e.g., a frequency which may be referenced to the AC power line assuming both the transmitter and receiver are on the same power grid, so that the area of the document scanned by the optical pickup means equals the overall area of the copy medium scanned by a writing mechanism such as a stylus in a given period of time. However, driving the transmitter drum and the receiver drum at the same angular frequency does not assure that the edge of the document will necessarily correspond with the edge of the copy since the initial position of the optical pickup means with respect to the edge of the document is not necessarily the same as the initial position of the writing mechanism with respect to the edge of the copy medium.
In U.S. Pat. No. 3,872,239 the positioning of the edge of the document relative to the optical pickup means and the edge of the copy medium relative to the writing mechanism is not required since the copy medium comprises a continuous loop of material. After a facsimile transmission, the edge of the copy medium may be appropriately located by cutting the closed loop of the copy medium at the proper margin location. Where a continuous loop of the copy medium is not utilized, it is necessary to synchronize the edge of the document relative to the optical pickup means and the edge of the copy medium relative to the writing mechanism before transmitting the information content of the document to the receiver.
In copending application Ser. No. 493,119, filed July 30, 1974, now abandoned in favor of continuation application Ser. No. 676,369 filed Apr. 12, 1976, a method and apparatus is disclosed for synchronizing the angular frequencies of the transmitter and receiver drums once synchronization of the edge or margin of the document relative to the optical pickup means and the edge or margin of the copy medium relative to the writing mechanism is achieved. This is accomplished by operating the transmitter at a constant frequency determined by a first crystal oscillator and operating the receiver at an initial variable frequency until margin synchronization is achieved and then generating the same constant frequency as generated at the transmitter but under the control of a second crystal oscillator after margin synchronization has been achieved.
At the present time, a substantial number of facsimile transceivers are in commercial use which utilize the closed loop concept disclosed in the aforesaid U.S. Pat. No. 3,872,239. These transceivers are all synchronized to an AC power line. Therefore, it is necessary for any transceiver receiving from a closed loop transceiver of this type to also be synchronized to the AC power line. However, at the present time there is no equipment available which permits synchronous operation on an AC power line while also providing a margin sychronization capability.
In the closed loop transceivers of the type disclosed in the aforesaid U.S. Pat. No. 3,872,239, synchronous motors are utilized to drive the rotatable drum. Such motors are extremely effective in maintaining synchronous operation will the AC power line frequency as long as there is no interruption in power. However, a brief interruption in the power can cause a loss of synchronization which can result in an illegible or at least highly distorted facsimile or copy.